Single Crystalline La0.7Sr0.3MnO3Molecular Sieve Nanowires with High Temperature Ferromagnetism

Abstract

Porous mixed-valent manganese oxides are a group of multifunctional materials that can be used as molecular sieves, catalysts, battery materials, and gas sensors. However, material properties and thus activity can vary significantly with different synthesis methods or process conditions, such as temperature and time. Here, we report on a new synthesis route for MnO(2) and LaSr-doped molecular sieve single crystalline nanowires based on a solution chemistry methodology combined with the use of nanoporous polymer templates supported on top of single crystalline substrates. Because of the confined nucleation in high aspect ratio nanopores and of the high temperatures attained, new structures with novel physical properties have been produced. During the calcination process, the nucleation and crystallization of ε-MnO(2) nanoparticles with a new hexagonal structure is promoted. These nanoparticles generated up to 30 μm long and flexible hexagonal nanowires at mild growth temperatures (T(g) = 700 °C) as a consequence of the large crystallographic anisotropy of ε-MnO(2). The nanocrystallites of MnO(2) formed at low temperatures serve as seeds for the growth of La(0.7)Sr(0.3)MnO(3) nanowires at growth temperatures above 800 °C, through the diffusion of La and Sr into the empty 1D-channels of ε-MnO(2). Our particular growth method has allowed the synthesis of single crystalline molecular sieve (LaSr-2 × 4) monoclinic nanowires with composition La(0.7)Sr(0.3)MnO(3) and with ordered arrangement of La(3+) and Sr(2+) cations inside the 1D-channels. These nanowires exhibit ferromagnetic ordering with strongly enhanced Curie temperature (T(c) > 500 K) that probably results from the new crystallographic order and from the mixed valence of manganese.